The petroleum industry is the largest consumer of energy in the nation, using 6.4 quadrillion BTU's in 1994. See Manufacturing Energy Consumption Survey 1994 (MECS) Energy Information Administration, U.S. Dept. of Energy, available at the URL “eia.doe/gov.emeu/mecs/mecs94/consumption/mecs5.html” Distillation operations constitute some 35-40% of the energy utilized, while hydrodesulfuration accounts for another 18%. The need for reducing energy costs to provide a more globally competitive industry while meeting environmental legislation has led companies to devise new strategies to achieve these goals. See Cumulative Impact of Environmental Regulations on the U.S. Petroleum Refining, Transportation and Marketing industries, American Petroleum Institute, October 1997.
Membrane pervaporation technology holds promise to significantly decrease petrochemical energy consumption. Pervaporation is characterized by imposition of a barrier membrane between a liquid and a gaseous phase, with mass transfer occurring selectively across the barrier to the gas side. Because of the unique phenomenon of phase change required of the liquids across the barrier, the process is termed pervaporation. Thus pervaporation membranes separate molecules on the basis of molecular interactions with the polymer in the membrane. This type of separation does not require intensive heat energy that is usually associated with distillation, since often a vacuum is applied to the gas side as a driving force. By having polymers with a specifically tailored architecture, these membranes may enable otherwise difficult separations and create opportunities for new separation techniques. Pervaporation membranes, either alone or in hybrid configurations with distillation or extraction, may provide optimal processes and additional latitude for their implementation. See R. Rautenbach and R. Albrecht, The Separation Potential of Pervaporation Part 2, Process Design and Economics, 25 J. Membrane Sci. 25-54 (1985).
However, commercial use of membranes in non-aqueous environments, such as those of petrochemical feedstreams, has only recently been realized. See G. Krishnaiah and J. Balko, Ultra-low Sulfur Gasoline ComplianceCosts with Davison Clean Fuels Technologies, Presentation at the National Petrochemical & Refiners Association Annual Meeting, San Antonio, Tex. (March 2003). The inability of pervaporation membranes to withstand long-term exposure to the moderately high temperatures has been an important issue. Excessive swelling, and chemical and temperature instability, have led to selectivity losses and failure. The need for a robust membrane material capable of withstanding exposure to organic liquids has been identified as a primary obstacle to the implementation of membrane technology.
The use of membranes to separate aromatic hydrocarbons from aliphatic hydrocarbons is generally known in the scientific and industrial community. For example, U.S. Pat. No. 4,115,465 teaches the use of polyurethane membranes to selectively separate aromatics from saturated hydrocarbons by pervaporation. U.S. Pat. No. 4,944,880 teaches the use of polyurethane-aliphatic polyester and polyamide-aliphatic polyester copolymers for the separation of aromatic and aliphatic mixtures. U.S. Pat. No.4,946,594 describes membranes for the separation of aromatic and aliphatic mixtures produced from reaction of an aliphatic polyester diol with a dianhydride and a diisocyanate. U.S. Pat. No. 5,128,439 describes the synthesis of high molecular weight saturated polyesters, and U.S. Pat. No. 5,177,296 describes a method for separating aromatic from aliphatic hydrocarbons using these polyesters. U.S. Pat. No. 5,138,023 describes a method for the synthesis of unsaturated polyesters, and U.S. Pat. No. 5,180,496 describes a method for separating aromatic from aliphatics using these polyesters. U.S. Pat. No. 4,828,773 describes polyurea/urethane membranes containing aliphatic polyester blocks for the separation of aromatic and aliphatic mixtures. U.S. Pat. No. 5,093,003 describes polyester-polyurethane block copolymers for aromatic aliphatic separations.
It would therefore be desirable for a more stable membrane for the separation of mixtures of aromatic and aliphatic organic.